The present invention relates to a direct-current to three-phase alternating-current inverter system (hereinafter referred to as “DC to three-phase AC inverter system”) in which a direct-current power source (hereinafter referred to as “DC power source”) is connected to a three-phase motor at a neutral-point thereof.
When the direct-current voltage (hereinafter referred to as “DC voltage”) is decreased in an inverter circuit for a three-phase motor, the inverter circuit requires a large current for obtaining the same output voltage as that outputted before the DC voltage is decreased. To meet this requirement, a switching element having a high current rating or a booster circuit may be arranged in the inverter circuit. If such measures are taken, the size of the switching element is increased, and a booster circuit is required to be added to the inverter circuit, so that the size of the inverter circuit becomes large thereby to increase the cost of the inverter circuit.
In order to solve the problem, a DC to three-phase AC inverter is proposed, for example, by Japanese Patent Application Publication No. 10-337047, in which the voltage of the DC power source connected to the neutral-point of the three-phase motor is boosted thereby to charge the capacitor, and the DC power of the capacitor is converted into the three-phase AC power to be supplied to the three-phase motor. In the DC to three-phase AC inverter system disclosed in the above Publication, both of the inverter operation and the boost operation may be accomplished by performing the boost operation in the region of the zero-voltage vector of the inverter operation. Thus, a large-sized switching element for large current application is not required for the inverter circuit. In the DC to three-phase AC inverter system, the zero-phase inductance of the three-phase motor may serve as a reactor for the boost operation, so that a booster circuit is not required to be added in the DC to three-phase AC inverter system other than the inverter circuit.
In the above-described DC to three-phase AC inverter system, if the voltage is not enough for driving the three-phase motor with the boost voltage ratio two or less, the inverter operation is performed through the overmodulation pulse width modulation control (hereinafter referred to as “PWM control”) when the amplitude of the voltage command becomes larger than that of the voltage of the capacitor. Referring to FIG. 3, in the overmodulation region T 1, the drive signal S1 for the switching element of the upper arm for the U-phase continues to be in a high level, and the switching element of the upper arm for the U-phase continues to be on when the inverter operation is performed through the overmodulation PWM control. In the overmodulation region T2, the drive signal S1 continues to be in a low level and the switching element of the U-phase upper arm continues to be off. In the overmodulation region T1, where the switching element of the lower arm for the U-phase is not on, energy is not charged in the inductance of the three-phase motor, so that the boost operation cannot be performed. In the overmodulation region T2, where the switching element of the upper arm for the U-phase is not on, the energy stored in the inductance of the three-phase motor is not discharged to the capacitor, so that the boost operation is not performed. The same is true of the V-phase and the W-phase. In the above DC to three-phase AC inverter system, when the inverter operation is performed through the overmodulation PWM control, the boost operation may fail to be performed, with the result the operation of the three-phase motor is performed in the limited range.
The present invention is directed to providing a DC to three-phase AC inverter system in which a DC power source is connected to a three-phase motor at a neutral-point thereof and which prevents that the operation of three-phase motor is performed in a limited range when the inverter operation of the inverter system is performed through the overmodulation PWM control.